The eyes of the motorsport world will gaze upon Paris tonight as a panel of FIA judges rule on the legality of the controversial 'double-decker' diffuser.

Stewards initially cleared the device's use prior to the Australian Grand Prix, allowing Brawn GP, Williams and Toyota to compete in the 2009 championship without altering their designs.

But Ferrari, BMW, Red Bull and Renault appealed the decision, claiming the use of the diffuser was an unfair exploitation of Article 3.12.5 of the FIA's technical regulations.

The rules state: Fully enclosed holes are permitted in the surfaces lying on the reference and step planes provided no part of the car is visible through them when viewed from directly below.

The key point of contention regarding this rule is that the 'Diffuser Three' refer to the holes in their diffusers as gaps, but Ferrari advisor Rory Byrne believes this interpretation is against the spirit of the FIA laws.

He told La Gazzetta dello Sport, “It's a rule set at least 14-15 years ago, and that for many years everyone interpreted in the same way. If you look at the Brawn car from underneath, you can see the suspension.”

The FIA's ruling has the potential to completely change the complexion of the championship, and according to Kimi Raikkonen, will be the deciding factor in this year's title race.

"The FIA's Court of Appeal will decide about the diffuser and this decision will have an enormous impact on the championship," Raikkonen said to Autosport.

"We're missing grip and downforce. You just need to analyse the performance in the three sectors at Sepang to understand that we're losing a lot compared to the best cars. You could see it especially in the middle sector where downforce is really crucial.”

There will be tears before bedtime for one group tonight. But which one?